This invention relates to cement and more particularly to a cement having high early strength characteristics.
The use of mixtures of portland cements and aluminous cements to provide a cement mixture which exhibits high early strength is known. However, such mixtures, in comparison to portland cement or aluminous cement alone, characteristically exhibit reduced ultimate strength as well as a tendency over a wide range of composition to flash set.
It is also known to add calcium sulfate to portland-aluminous cement mixture. Spackman et al. U.S. Pat. No. 903,019 states that the addition of 1-3% calcium sulfate to a portland cement containing calcium aluminate acts as a controlling agent -- apparently to control setting time. Such mixtures have also been used to provide cements which expand upon setting. For example, Schenker et al. U.S. Pat. No. 2,465,278 adds a mixture of calcium hydrate [Ca(OH).sub.2 ], calcium sulfate, and an aluminous compound to a portland cement mixture to impart expansive properties thereto. While such cements exhibit certain advantages in the casting of reinforced structures or when prestressing is desired they are not always suitable for use in patching type operations such as, for example, on a highway, because of the expansion which can cause buckling. In addition, such cements are not usually known to possess high strength, suffering either from a lower early strength or a somewhat lower subsequent strength. Thus, for example in Klein U.S. Pat. No. 3,155,526 and Klein U.S. Pat. No. 3,251,701, suitable data is offered with regard to the expansion characteristics of a resultant cement but very little is said regarding the strength of the cement. H. Lafuma, in a paper entitled "Expansive Cements", published in the Proceedings of the 3rd International Symposium on Chemistry of Cement (1952) indicates (on page 584) that combinations of high alumina cement and calcium sulfate with portland cement produce "remarkable strengths at early ages". However, Lafuma does not indicate what level of strength is reached at what period of time nor what ratios one should use to achieve such results, since he appears to prefer the use of a clinker which contains calcium silicate as well as calcium aluminate and calcium sulfate. Lafuma further mentions (on page 585) that the composition of non-shrinking cements is very similar to that of expansive cements and presents data showing the amount of SO.sub. 3 and sulpho-aluminate necessary to provide the expansion that he desires. However, Lafuma does not indicate how one may obtain a dimensionally stable (non-expansive) cement which will exhibit high early strength as well.
It is also known to combine tricalcium silicate (one of the ingredients in portland cement) with calcium aluminate and calcium sulfate. Sadran U.S. Pat. No. 3,645,750 describes such a mixture and states that it has good early strength without quantifying either the time or degree of strength. Furthermore, Sadran indicates that his basic binary composition of tricalcium silicate and aluminous cement attained its peak strength more gradually and actually showed a lower strength for the first 24 hours than reference aluminous cements.
The use of sulfo-aluminate cements -- without portland cement -- is also known. Armstrong et al. U.S. Pat. No. 3,147,129 describes such cements and states that they do not appear to give very high early strengths as opposed to ordinary aluminous cement. Armstrong et al. propose the addition of fluorine to the sulfo-aluminate cement to obtain higher strength. However, the main use of such blends still appears to be to obtain an over-all expansion property of the cement.
There remains, however, a need for a cementitious mixture which will be substantially neither expansive nor exhibit shrinkage and which may be used to provide a product exhibiting high early strength as well as sufficient ultimate strength. Such a product, for example, can be used in the patching of highways, particularly main arteries such as Interstate highways or turnpikes which cannot be closed for long periods of time sufficient to permit ordinary concrete to develop satisfactory ultimate strength such as, for example, 28 days. Similarly, the patching of airport runways necessitates the use of a cementitious material which will exhibit sufficient high early strength to permit the use of the runway within a short period of time after the patching without, however, sacrificing the ultimate strengths obtainable with ordinary cements.